This invention relates to an element having a periodic structure capable of guiding electromagnetic waves. More particularly the invention relates to a element in which the periodic structure provides (i) propagation and stop bands (i.e. frequency bands in which a wave propagates without attenuation separated by bands in which the wave cannot propagate); and (ii) guidance of waves having a propagation velocity inferior to that of light.
A structure that offers the above-noted characteristics can allow the propagation of an electric field with a strong axial component which, during propagation at low speed, can interact with an electron flow.
The inventive periodical structure element, owing to the first property mentioned above, can be applied to microwave filters and, owing to the second property, it may find application in electric-charge accelerators and in microwave tubes.
For illustrative purposes, the invention will be described only with reference to the second property due to the interesting developments which it offers when applied to travelling wave tubes and to electric-charge accelerators.
Such type of tube is in principle formed by the following items:
an electron gun that emits an electron beam at a prefixed speed; PA0 a waveguide to input an electromagnetic wave into the tube; PA0 an element with the task of propagating the electromagnetic field generated by the above-mentioned wave; PA0 an output waveguide to pick up and make available the output signal; and PA0 a collecting electrode that gathers the electrons of the beam above and puts them back into circulation.
A tube of the type described forms an amplifier based upon the interaction between the electron beam and the electromagnetic field originated by the signal to be amplified, which propagates at a speed slightly lower than that of the electrons. The alternating electric field of the wave causes a modulation of the electrons of the beam and this results in a modulation of the density of the beam itself. If the velocity of the electron beam is greater than the speed at which the electromagnetic field propagates along the structure axis, the electrons find a systematic perturbation induced onto their motion; consequently, the electrons slow down, releasing energy to the electric field, which is in turn passed onto the wave that propagates along the structure. As the waveguide for output signal collection is set at the end of the structure, it follows that the electromagnetic wave picked up there has a higher energy level than the wave input to the tube.
The amplifying effect of the tube is a function of the coupling between the electromagnetic field and the electron flow. To keep such coupling within values that provide a reasonable amplification level, it is necessary to shape the guidance element so that a part of such electron flow (or beam current) is intercepted, causing generation of heat.
In tubes with coupled cavities of known configuration, there are upper limits to the thermal dissipation capability that are due to the large azimuthal dimensions of the coupling posts which cause an increase of the thermal resistance between the areas subject to maximum heat and those connected directly to the cooling circuit.
An object of this invention is to provide an element for guiding electromagnetic waves that minimizes to a considerable extent the drawbacks mentioned above. The invention presented offers the possibility of introducing distributed losses along the entire structure, making use of wires with a resistive surface which, when the element is used in a travelling wave tube, provides for the elimination of undesired oscillations.
The electromagnetic-wave guiding element, made in accordance with the invention, will therefore offer a structure in which the connecting posts between cavities are replaced by circular holes having a reduced cross section, and the coupling among cavities is obtained by means of conductive wires which cross the cavity.
Such structure, as in the case of already known coupled cavity structures, is periodic and it therefore presents all the characteristics mentioned above.